Separation of c8 aromatic hydrocarbons

ABSTRACT

PARAXYLENE IS RECOVERED FROM A MIXTURE OF XYLENES AND ETHYLBENZENE BY SEPARATING THE MIXTURE INTO TWO STREAMSONE WHICH IS RICH IN ETHYLBENZENE AND ORTHOXYLENE AND ANOTHER WHICH IS RICH IN METAXYLENE-AND SEPARATELY ISOMERIZING THE TWO STREAMS UNDER DIFFERENT CONDITIONS TO OBTAIN A MAXIMUM AMOUNT OF PARAXYLENE. D R A W I N G

Oct. 16,1973 o. G. STENMARK E 3,765,237

SEPARATION OF C8 AROMATIC HYDROCARBONS Filed July 23. 1971 ETHYLBENZENE v LIGHT a HEAVY 32 PRODUCTS |40 3| 3lo l4 f 5 I6 FRACTIONAL 7 w DISTILL ATION l7 HIGH SEVERITY 7 FEED N ETHYLBENZENE PARAXYLENE H 33 METAXYLENE ORTHOXYLENE l2 29 SEPARATION l9 ZONE .43 26 PARA- META XYLENE LOW SEVERITY ISOMERIZATION \"IB P-XYLENE PRODUCT 28 -2| A P\ FRACTIONAL DISTILL ATION LIGHT a HEAVY PRODUCTS 20 3' O'XYLENE INVENTORS.

DONALD G STENMARK, ROBERT D.WESSELHOFT,

United States Patent 3,766,287 SEPARATION OF C AROMATIC HYDROCARBONS Donald G. Stenmark, Houston, and Robert D. Wesselhoft,

Baytown, Tex., assignors to Esso Research and Engineering Company Filed July 23, 1971, Ser. No. 165,600 Int. Cl. C07c /08 U.S. Cl. 260668 A 16 Claims ABSTRACT OF THE DISCLOSURE Paraxylene is recovered from a mixture of xylenes and ethylbenzene by separating the mixture into two streams one which is rich in ethylbenzene and orthoxylene and another which is rich in metaxyleneand separately isomerizing the two streams under different conditions to obtain a maximum amount of paraxylene.

BACKGROUND OF THE INVENTION (1) Field of the invention The present invention is directed to separation and production of C aromatic hydrocarbons. More particularly, the invention is concerned with the production of paraxylene from its isomers. In its more specific aspects, the invention is directed to subjecting different fractions of C aromatic hydrocarbon isomers in non-equilibrium amounts to different conditions of isomerization to separate and produce a maximum amount of paraxylene from a mixture of C aromatic hydrocarbons.

(2) Description of the prior art It has been well known heretofore to isomerize a mixture of C aromatic hydrocarbons and to separate paraxylene therefrom by crystallization or by adsorption selectively on an adsorbent such as a molecular sieve. It has also been known to separate ethylbenzene and orthoxylene from the C aromatic hydrocarbon mixture and isomerize the metaxylene and recover paraxylene by crystallization and the like.

Ethylbenzene has been recovered by distillation from a C aromatic hydrocarbon mixture, as well as orthoxylene, and the metaxylene isomerized to paraxylene subsequent to removal of the naturally present paraxylene with recycle of the unreacted isomers. Such processes are disclosed in British Pat. 1,198,592 published July 15, 1970, and in US. Pat. 3,522,153. A similar operation is described in German application 1,905,650 open Aug. 20, 17970. In the present invention the orthoxylene as well as metaxylene both form a source for paraxylene and the ethylbenzene is converted effectively to lighter and heavy which heretofore was not taught or obvious.

SUMMARY OF THE INVENTION The present invention may be briefly described and summarized as a method for separating and producing paraxylene from a feed mixture with its isomers including ethylbenzene, metaxylene, and orthoxylene in nonequilibrium amounts, wherein the mixture is subjected to fractional distillation to obtain a first distillate rich in ethylbenzene and containing smaller amounts of metaxylene, paraxylene and orthoxylene and a second distillate rich in metaxylene and containing smaller amounts of paraxylene, orthoxylene and ethylbenzene. Orthoxylene is recovered as a bottom fraction and is admixed with the first distillate which is then isomerized under severe isomerization conditions over an isomerization catalyst. The second distillate has paraxylene substantially removed from it, and the remainder is then isomerized under less severe conditions than those to which the first distillate is subjected. The isomerized products are combined and may be admixed with the feed mixture and the cycle is repeated with the result that only paraxylene is produced as a desired product, the ethylbenzene and ortho xylene being recycled to extinction by virtue of using more severe isomerization conditions for the ethylbenzene and orthoxylene. Optionally, if desired some or all of the ethylbenzene and orthoxylene may be recovered as products. Ordinarily the feed mixture may not be mixed with the isomerizate and the product streams recycled in block fashion.

BRIEF DESCRIPTION OF THE DRAWING The present invention will be further described and illustrated by reference to the drawing in which the single figure is a flow diagram of a preferred mode and embodiment.

DESCRIPTION OF THE PREFERRED MODE AND EMBODIMENT RELATIVE TO THE DRAWING Referring now to the drawing, numeral 11 designates a fractional distillation tower provided with internal vapor-liquid contacting means such as bellcap trays not shown and heating means and other appurtenant means also not shown for separating one close boiling xylene from its isomer, in this instance ethylbenzene from metaxylene and other isomers. Tower 11 has a line 12 leading into it by way of which a non-equilibrium feed mixture of ethylbenzene, ortho, meta and paraxylene as Well as unreacted products from the isomerization zones are charges. A fraction comprising paraxylene, orthoxylene, and metaxylene is removed by line 13 while ethylbenzene is taken overhead by line 14a and may be either discharged from the system by opening valve 31 in line 31a and closing valve 32 or added into line 14 by opening valve 32 and closing valve 31 and introduced thereby into isomerization zone 15 operating under high severity conditions which are given hereinafter. Zone 15 comprises separation facilities for removing light and heavy fractions originally present or formed in zone 15. These separation facilities may suitably be fractional distillation facilities and the like. The light and heavy fractions are shown removed by line 16 but usually will be removed by separate lines. The eflluent product from zone 15 comprised of paraxylene is discharged by line 17 into feed line 12 for recycling to tower 11 but may be treated in blocked operations as will be described.

The bottoms fraction removed from tower 11 by line 13 comprising ortho, meta, and paraxylene are charged thereby to fractional distillation tower 18 which is similarly equipped to tower 11. In fact, towers 11 and 18 may be a single fractional distillation zone comprised of one tower but is shown as two towers for the sake of convenience. In any event, a paraxylene-metaxylene stream is removed by line 19 and an orthoxylene stream by line 20 which may be recycled to line 14 by line 21 with valve 22 in the open position. Orthoxylene may be discharged from the system and recovered as a product by closing valve 22 and opening valve 23 in line 20.

The paraxylene-metaxylene fraction flows by line 19 into a separation zone 24 which may be a crystallization zone operated under suitable low temperature conditions which are well known to crystallize paraxylene at low temperatures and allow it to be removed by means 25'; zone 24 may also comprise a filter or centrifuge and means 25 may include heating means to allow recovery of the paraxylene as a liquid. Zone 24 may also be an adsorption zone which selectively separates paraand metaxylene. Suitable adsorbents are known for separating paraand metaxylene and further details thereof are not given here.

The metaxylene from which the paraxylene is substantially removed in charged to an isomerization zone 27 I 'which gopera'tes under low severity conditions which are i converted to enhance. the production of v aluableparaxy 919,,"a a Amir r al..U.S. 3,449,456. I

. hydrogen: pressure/of at 7 catalyst temperature increased atirateoi about 100? Fsper houruntil 11500115."

1 aqueous solution of ammonium paramolyhdate to provide 'a catalyst. containing from about 3% to about by weight. M003 ongdry basis.

given herein with the metaxylene being converted. largely to paraxylene. Any light and'heavy fractions-producedin the..isornerizationoperation .ornormally present in theif stream introduced by 1ine12 6 are/removed by "linef'28 whichordinarily' will be two lin'es but which is shown solely for convenience as one line'siince" zone 27 may,.'lile' 1 MATER L BALA CE.with innr'mcrromrros AND? PARALLEL rsonssrz rron EXAMPLE 1 To illustrate the present invention furthenfreference: is 3 made to the following material balance where the composition of the various streams is giyenyas designated by. the numerals identifying the several lines in theldrawing for 80% ethylbenzene recoveryat 80%] purity: Y

' streamnii. -J

: Relative to 100 parts feed.

zone 15, comprise fractional distillation means, adsorp i 20 tion means,.other separation means, and, the like. Thel'; I product is then introduced. by line29i11to llnetllior admixture with the: feed. introduced into line 12 by line "30;

It willthus be apparent, from the'foregoing' descrips on.

that a simple methodwhich has not been taughtinnof obvious from the art has been devised lwherepmetaxylene, and orthoxylene containing .ethylbenzene. are separately i lenewhichis recovered foru'sc a's de'sired'.

The isornerization conditions are shown in the .follow-v ing tabulation" for various types of catalysts. 'TheMoOg on silicasalumina catalysts are. also. desieribedin. the patents to Amir 'US. 3.484,3,85-,=Amir et :al..U.S..3,4l0,-

' RANGE QF'ISOM'ERIZATIONCONDITIONS. p [Range of Low and High Severity Conditions],

' V 4 Catalyst 7' M603 0 having the 1 having the composition compositionj alumina listed below 2 listed below a J v 1 Temp., 700 950 aroeoso fi Press. p.s.i.g 150-500 I 150-500 Liquidieed rate, v./v./hr 0.1-10 0.1710 s H: to feed,-mole ratio s... 2:ll5:1 2-15:1-. 3':1-'15:1 H purity o, mole -percent. -100 50-100 50-100" N1, mole percent, 01:4;0 0.1-4 1 0.1-.4

Shaped silica-alumina molydbenumecontalningcatalyst exposed o, i

7 least 100 'p.s.i.g. followed by heating to a temperature otabout:650, to nogreaterthan 850 Riorat least 10 hours. I

Synthetic zeolitebase having .a. moi ratio of oxides aswfollows:

X has a value of about 0001 to about 0.2 and Y may; be any; value from, 0 to about-10 and Me represents an alkali selected from the groupconsisting of I alkali= metals and. their mixtures- Heat treatment of base from about l',000 to 1,500 and 'held'aflatter tempera. ture iorabout .1 to 24 hours. Steamed, startedatabout'700 F. with is reached; cooled to ambient temperature and base exchanged with Jan 7 3 Synthetic zeolite base having a: mol s asfollows: 1.0=l=0.1,(1z) MezO :(XXN (GI-I04) 2Q] -Al2Oa:6;5'=l:1.0 SiQ ,JYHaQ where X has, a value of about 0.001" to about 0.2, Y may be any value from 0 to about 10 and Me represents either potassium or an alkahjmetal' selected from the group consisting of potassium, sodium andmlxtures thereof. The unit cell of the zeolite base is hexagonal with an A axis 13.31 'A. and 0 axis 7.59 A.: that is with the 0 axis being half that of en'onite which may be used. The zeolite base is an ion exchanged with an ammonium salt (NHrCla, (NHQzSOr, or NHrNOa) in an aqueous solution to form the ammonium form and heated to liberate N113 and form the hydrogen base which is calcined at a temperature from about 700 F. to about 1,500 F. and held for about 1 to 24 hours in presence of steam which is started at 700 with temperature increasedat arate of about 100 F. per hour. It is then cooled to ambient temperature and impregnated or exchanged with an ammoniacal solution of palladium or platinum chloride to give a catalyst containing about 0.1 to about 1.0% by weight of metal.

No'rE.-- Impurities include 01 to C5 light hydrocarbons. Low severty conditions: The low severity conditions are in the same range as the high severity conditions but generally will differ in at least one respect or another such as liquid feed rate, temperatures, concentration of the various components in the streams exposed to the isomenzation conditions, and the like.

tions of Example 1 Strearn P rcent; j

' Ethylbonzen'eud 20.0 32:9 7 85.2. Paraxylene 1 9.0 14.9 9.0 Met-axylene 42.0 1 35:5 38.0 Orthoxylene. 19.0 16;7 17.0,. Total new 5100.0 seas 837.0.

From the, foregoing tabie, it'i's ciearthat the 'ethyl i benzene and'orthoxylene are substantially converted and pure paraxylene is: obtainable: with high; rccofverie'sot the j ethylbenzene and'orthoxylate forchargin'g to the several isomerization. zones the conditions; of which are indicated;

in thetable belowwith ,recycli ng to; extinction-Q oat r sa ooi a silica-.alupina] Isomerizetien: conditions;

Highse i Lowse, verity verity Operation (15)" (27) EXAMPLE ,2,.

.. 1 ,By way of comparison iwitliiExamplee l," referenceiis .made totheiollowing materialjbalance andthedrawing; 1

whereby the fractional distillation means (zones 11 and 18-)=are not employedtby opening value. 34 'in'lin'e34a and closing valve. 33) fwhereby oniyf one. isomerization;

step (zone27) is employedat theflow ,severityieondb Relative. to parts: I

From the foregoing table it is clear I that stream 19.

which flows into the paraxylene separation zone 24 is overly large compared to the same stream in Example 1 and that the recovery of paraxylene is also lower. In addition, stream 26 which flows. into the isomerization zone 27 is overly large compared to the combined streams, 2'6

and 14, whichflow into both isomerization zones (27'. and 15) in Example 1.

EXAMPLE 3 by only one isomerization step (zone 27) is employed at the high severity conditions of Example 1:

Relative to 100 parts feed.

From the foregoing table it is clear that the recovery of paraxylene is substantially lower compared to Example 1, and stream 19 which flows into the parax'ylene separation zone 24 is also larger compared to Example 1. From the foregoing examples, it is clear therefore that in a recycle process, the recovery of paraxylene from a mixture of xylenes and ethylbenzene is enhanced by separating the recycle mixture into two streams-one which is rich in ethylbenzene and orthoxylene and another which is rich in metaxylene-and separately isomerizing the two streams under different conditions to obtain a maximum amount of paraxylene.

While exemplary feed compositions are given, it is to be understood that we are not to be limited to the particular concentrations of any one component which are given by way of illustration and not by way of limitation. The nature and objects of the present invention having been fully described and illustrated and the best mode and embodiments contemplated set forth, what we wish to claim as new and useful and secure by Letters Patent is: 1. A method for isomerizing a xylene fraction containing ethylbenzene, orthoxylene, metaxylene, and paraxylene in non-equilibrium amounts which comprises the steps of:

fractionally distilling said fraction to obtain a first distillate rich in ethylbenzene and containing paraxylene, metaxylene and othoxylene, and a second distillate rich in metaxylene and containing paraxylene, orthoxylene and ethylbenzene; separately subjecting said first distillate to isomerization conditions in an isomerization zone in the presence of an isomerization catalyst to convert said first distillate to a first product enriched in paraxylene;

separately subjecting said second distillate to a separation operation to recover paraxylene therefrom and to obtain a metaxylene enriched fraction; separately subjecting said metaxylene enriched fraction to isomerization conditions in an isomerization zone in the presence of an isomerization catalyst to convert selectively said metaxylene fraction to a second product enriched in paraxylene; and

subjecting said products to separation to recover said paraxylene.

2. A method in accordance with claim 1 in which the first and second products are combined.

3. A method in accordance with claim 1 in which the first and second products are combined and then subjected to distillation and separation to recover said paraxylene.

4. A method in accordance with claim 1 in which the separation operation is a crystallization step.

5. A method in accordance with claim 1 in which the first distillate is subjected to high severity isomerization conditions and said metaxylene enriched fraction is subjected to low severity isomerization conditions.

6. A method in accordance with claim 5 in which the low severity isomerization conditions include the following conditions:

Temperature, F. 750.

Pressure, p.s.i.g 250.

V./v./hour 2.0.

Catalyst MnO on silicaalumina.

Hydrogen to feed, mole ratio 8.5/1.

Hydrogen purity, mole percent 85.

Nitrogen, mole percent 0.2.

and the high severity isomerization condition includes the following conditions:

Temperature, F. 750. Pressure, p.s.i.g 250. V./v./hour 0.67. Catalyst M00 on silicaalumina.

Hydrogen to feed, mole ratio 8.5/ 1. Hydrogen purity, mole percent 85. Nitrogen, mole percent 0.2.

7. A method in accordance with claim 1 in which said low severityisomerization and said high severity isomerization are conducted in the same isomerization zone but at different times.

8. -A method in accordance with claim 1 in which the isomerization conditions include a temperature within the range of about 500 F. to about 950 F. and a pressure within the range of about to about 700 p.s.i.g, and a liquid feed rate of about 0.1 to about 16 v./v./hr.

9. A method in accordance with claim 1 in which the isomerization catalyst is M00 on silica-alumina.

'10. A method in accordance with claim 1 in which the isomerization catalyst is M00 treated oifretite.

11. A method in accordance with claim 1 in which the isomerization catalyst is palladium on treated ofrretite.

12. A method in accordance with claim 1 in which the isomerization catalyst is platinum on treated erionite.

13. A method in accordance with claim 1 in which the isomerization catalyst is platinum on treated offretite.

14. A method in accordance with claim 1 in which the isomerization catalyst is palladium on treated erionite.

15. A method in accordance with claim 1 in which the fractional distilling is conducted in separated distillation columns.

16. A method in accordance with claim 1 in which the fractional distilling is conducted in a single distillation column.

References Cited UNITED STATES PATENTS 3,449,456 6/1969 Amir et a1 260-668 A 3,584,068 6/1971 Jackson et al. 260668 A 3,384,676 5/1968 Lester 2'60668 A 3,582,497 6/1971 Best et a1. 260-455 Z 3,370,099 2/1968 Plan-k et a1. 260-668 A 3,636,121 1/1972 Stine et a1. 260668 A 3,636,180 1/1'972 Broughton 260-6 68 A CURTIS R. DAVIS, Primary Examiner U.S. Cl X.R. 

